This invention relates in general to stripline filters and more particularly to an improved stripline filter apparatus and method for making the same wherein performance characteristics are optimized and maintained during the fabrication process.
Stripline filters are of course known in the art. They differ from microstrip structures in that they include top and bottom ground planes sandwiched over a body of dielectric material in which a resonant structure is embedded. Normally, there is an upper dielectric body and a lower dielectric body. Typically, the two dielectric bodies are provided with image circuit metallization on their interior surfaces of the base and top which are then joined and permanently attached together.
However, provision for connection to the internally positioned resonator structure must also be made if the resultant apparatus is to have utility. Conventional interconnection technology until now has involved the application of pre-formed "paddle" leads, or alternatively, "clip-on" leads. A major disadvantage of the conventional paddle lead is that the mechanical strength and integrity of the attachment is primarily dependent on the quality of the metal adhesion to the surface of the dielectric body of the filter. For the clip-on lead, a cut-out in the overall sandwich assembly is required to accommodate the associated lead. This cut-out requirement wastes a substantial portion of a module edge of the filter assembly and mechanical strength nevertheless leaves something to be desired. There are other disadvantages as well which will be discussed in more detail subsequently herein.
A number of dielectric materials have been used in the past. A plastic or resilient material, such as teflon, has been employed in the past and enjoyed widespread usage. After metal definition, a thin plastic sheet is utilized in between the two halves, i.e., base and top, which are then pressed together under sufficiently high pressure and heat until welded together. This forms a reasonably well assembled and reliable filter. However, it is not suited to applications where size and volume constraints are imposed, say, for example, in a miniaturized, hand-held radio pager or transceiver. This is because of the relative low dielectric constant of the teflon, on the order of two or three. For such miniaturized, portable applications, a ceramic substrate is advantageously utilized which has a dielectric constant of many orders higher, which then results in a resonant structure of far less size. That is, the quarter wave dimension of the resonator structure is a function of the dielectric value.
The perceived disadvantage of ceramic is that it cannot be simply pressed together in the manner permitted with the somewhat more resilient teflon based filter. With the ceramic substrate, the two filter halves typically are soldered together by known solder technology. However, with this soldering process, the thickness of the gap in the center of the sandwiched assembly, and hence the total thickness, which is a critical parameter of the associated filter, is determined by the amount of solder applied and the distribution thereof as well as solder wetting and other factors. Typically, this gap between filter sections may vary between three to twelve mils. This variation is undesirable as it effects a corresponding variation the bandwidth of the filter as well as other operating aberrations.